The present invention relates to a method for determining physiological characteristics produced by the functioning of the heart, such as stroke volume and cardiac output. The method of the present invention may provide absolute, as contrasted to relative, values of these characteristics.
In many instances, such as prior, during, or following major surgery, it is important to determine certain physical characteristics of a patient's heart. For example, it may be necessary or desirable to determine the volume of blood being discharged from the patient's heart. The volume may be expressed as the stroke volume (SV), which is the volume of blood discharged during each contraction or beat of the heart, or the cardiac output (CO), which is the volume of blood discharged over a given period of time, such as a minute. Or, it may be necessary or desirable to know atrial and/or ventricular ejection times in order to assess the contractility of the heart muscle.
The most direct way to measure these quantities is to use a pulmonary artery catheter. Such a catheter is surgically positioned inside the heart in a pulmonary artery catheterization procedure. However, such positioning is a highly invasive, labor intensive procedure which requires a high level of surgical skill and which places a patient at considerable risk. For these reasons, attempts have been made to develop less invasive, or non-invasive, methods for determining the amount of blood discharged from a patient's heart.
Approaches taken to this end include the following. Some methods use changes in the electrical impedance of the thorax, as measured by electrodes placed on the patient's skin to determine cardiac functioning. See U.S. Pat. Nos. 5,782,774; 5,685,316; 5,469,859; 4,870,578; 4,450,527; and 4,437,469. Other methods employ ultrasonic probes. For example, such probes may be placed in the suprastenal notch or in the esophagus to measure blood flow in the aorta. See U.S. Pat. Nos. 5,575,289; 5,085,220; 5,052,395; and WO 98/51212. Another approach is to apply various algorithms to the blood pressure waveform to determine cardiac output. See U.S. Pat. Nos. 5,876,347; 5,647,369; 5,584,298; 5,535,753; 5,390,679; 5,265,615; 5,241,966; 5,183,051; 5,101,828; 5,025,795; 4,676,253; 4,137,910 and WO 97/47236 and Japanese Patent Publication 10-094528. Dye dilution techniques have also been employed in which a dye is injected into the blood stream and a sensor detects the dye concentration in the blood after the dye has passed through the heart. See U.S. Pat. Nos. 5,494,031 and 5,458,128. In a technique, commonly called the Fick method, CO.sub.2 is measured in the air expired by the patient and used to make a determination of the volume of blood discharged by the patient. See U.S. Pat. Nos. 5,836,300 and 5,043,576 and WO 98/26710 and WO 98/12963.
However, a problem with one or more of the foregoing methods is that they may produce inaccurate results due, for example, to the use of less than correct assumptions in deriving an algorithm for determining volumes from the shape of a blood pressure waveform. Further, the methods determine only a relative blood volume value. In order to produce an absolute blood output value, a calibration value obtained by a pulmonary artery catheter is required.